A positron-emitting isotope is administered to the patient and some of it reaches the brain through the blood.

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What happens after the isotope reaches the brain?

Each positron emitted travels no further than a few mm before annihilating eachother. 2 gamma photons are produced, and are sensed by detectors linked to computers.

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What happens after the detection reaches the computers?

Gradually, an image is built up from the detector signals.

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When does positron emission take place?

When a proton changes into a neutron in an unstable nucleus with too many protons.

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How are positron emitting isotopes produced?

By placing a stable isotope in the path of a beam of protons, causing them to absorb extra protons and becoming unstable positron emitters.

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When, and who by, was antimatter predicted?

In 1928, by Paul Dirac, an English physicist. This was before the first antiparticle, the positron, was discovered.

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What did Einstein show about the mass of a particle and the speed it travels?

The mass of a particle increases the faster it travels, leading to the famous equation E=mc2. Also, he showed rest mass corresponds to rest energy locked up as mass. He showed rest energy must be included in conservation of energy.

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Dirac predicted that antiparticles could what?

Unlock rest energy, whenever a particle and a corresponding antiparticle meet and annihilate.

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Dirac's theory of antiparticles predicted that, for every type of particle, there is a corresponding antiparticle which what?

Annihilates the particle and itself if they meet, converting their total mass into photons, has exactly the same rest mass of the particle, and has exactly opposite charge to the particle if the particle has a charge.

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What happens in pair production?

A photon with sufficient energy could suddenly change into a particle-antiparticle pair, which would then separate from eachother.

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What is the definition of an electron volt?

The energy transferred when an electron is moved through a PD of one volt.

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Why are two photons produced in annihilation?

Because a single photon would only take away momentum which isn't allowed, as no outside forces act.

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What is the minimum energy of each photon produced equal to?

E0, the rest energy of the electron.

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In pair production, the minimum energy of photon needed is equal to what?

2E0.

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When can we see the path of alpha and beta particles?

Using a cloud chamer; a small, transparent container containing air saturated with vapour and made very cold.

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How was the positron discovered?

Carl Anderson knew a positive particle would deflect in a different direction to a negative one, and when one beta particle was deflected in a different direction, the positron was discovered.